The present invention relates to a process for the automatic control of the temperature of a food product continuously heated by the direct injection of steam, where the flow of the food product through the injector may be varied.
In its simplest form, a steam injector consists of a pipe through which the product feed stock is pumped at a constant rate and steam is introduced through a jet or orifice plate from a high pressure supply line which can be modulated by a valve. A sufficient minimum length of pipe downstream from the injection point is required to ensure that thorough mixing of the feed stock and steam has occurred before an accurate measurement of the resulting temperature can be obtained. This essential delay before measurement means that any alteration in conditions which causes a variation in the final product temperature will not be detected for approximately five seconds after its occurrence and before any corrective action can be taken, for example, by means of traditional control technology using a three-term control algorithm in the feed-back loop. Any gradual change in conditions such as a slow increase in feed stock temperature can easily be compensated for this method. However, sudden changes, for example, caused by switching to a second feed stock batch at an appreciably higher or lower pre-injection temperature, can cause considerable disruption to the smooth running of the system and runs the risk (where the heating is being used to sterilise the product) of non-sterile product reaching the filler header tank. The signal from a pre-injection thermocouple could be used to compensate for sudden temperature variations of this nature using a conventional controller, but to ensure that the feed-forward compensation was applied at the correct moment in time, the position of this thermocouple would be dependent on the feed stock flow rate and would physically need to be moved if the feed stock flow rate were changed.
In practice, in an industrial situation, the demand for heated product by the filling line will vary depending on the number of breakdowns experienced on that line and it is advantageous to limit the amount of product which is stored hot in the header tank so as to prevent thermal degradation of the food product, for example, by hydrolysis of the thickening agents or from discolouration, browning, Maillard reactions, off-flavours or burn-on. This limitation may best be achieved by varying the rate at which the product is injected to the required temperature.
Soviet Inventor's Certificate No. 785,353 describes a method for the automatic control of a process for the production of alcohol by cooking starch-containing material with steam, which provides for measurement of the actual temperature of the mass after the secondary heating contact head, and regulation of the steam supply, characterised in that, for this purpose of improving cooking quality, the set value for the steam throughput is determined in dependence on the actual temperature and throughput of the mix, and the actual and set temperatures of the mass after the secondary heating contact head, while the supply of steam is regulated in dependence on deviation between the current and set values of steam throughput. However, since there is no feed-back indicated or mentioned from the feed stock flow rate sensor to the feed stock pump in the Soviet process, it is not possible to implement automatic feed stock flow rate control with this equipment.
When direct steam injection is used for the sterilisation of a food product in a continuous process it is necessary to have extremely precise control of the temperature and holding time at temperatures appreciably above 100.degree. C. and pressures well above atmospheric to avoid under- or over-processing giving the associated disadvantages mentioned above. In order to achieve this, we have found that, unlike the Soviet process, it is essential to have the means for the automatic control of the feed stock flow rate.
In any case, this Soviet process does not relate to a sterilisation process. In the description of the apparatus, the absence of any pressure regulating valve or orifice before the steam separation vessel would make the operation of the plant much above 100.degree. C. or much above atmospheric pressure impossible and thus this process could not be used in sterilisation applications. Any back-pressure to cause operation above atmospheric pressure would be generated by the frictional forces opposing the pumping operation through the pipes, plus the static head (if any) by which the product would be raised by the pumping operation. The holding time would depend on the feed stock flow rate, but means for automatic control of this is not really necessary in the Soviet process in which, for filling large vessels, the feed stock flow rate, which could be affected by the viscosity of the product and the back-pressure in the line after steam injection, would ideally be maintained constant over the widest possible range of viscosities and back-pressures by the use of a fixed speed, positive displacement pump.